Numerous industrial and natural processes involve the flow of thin liquid films. Common applications include liquid paints, floor wax, and wood sealants. In many of these applications, it is desirable to have a uniform (level) coating. Freshly applied liquid films, however, normally have uneven surfaces. These uneven surfaces, along with the dynamics of film drying, can produce coating defects such as orange peel or cratering (i.e., formation of surface blemishes, usually in the form of small round patches).
There is wide industrial interest in reducing and eliminating coating defects. Despite the interest in reducing coating defects, however, finding particular additives that are capable of serving as leveling agents remains an empirical exercise. In general, while some leveling agents are surfactants, the ability of any particular surfactant to lower the surface tension of a solvent or formulation has proven to have little predictive value in determining whether that surfactant would be useful as a leveling agent to reduce coating defects.
A useful leveling agent should be capable of maintaining that surface energy at a nearly constant value during drying. However, drying behavior of a two component system of resin or film-forming polymer and solvent may lead to surface gradients in surfactant that is present as reported, for example, by D. E. Weidner in “Role of Surface Tension Gradients in Correcting Coating Defects in Corners” in Journal Of Colloid And Interface Science, vol. 179, pp 66-75 (1996). For example, FIG. 10 of that paper reports that under some circumstances weaker surfactants may result in coatings with greater thickness at the edges, and hence more level, than if stronger surfactants are used.